Direct radiating Electronically Steerable Phased Array Antennas (ESAs) are generally comprised of multiple individual antenna elements spaced in a grid pattern on a flat or curved surface, the combined energy of individual elements forming the antenna. Steering of the antenna is accomplished by electronically adjusting the time delay or phase shift on individual elements in such a way that, for example, the energy received by each element from a plane wave in a selected direction combines coherently, whereas the energy in other directions does not. This process, commonly referred to as beamforming, is the fundamental basis for the ESA concept.
The pointing accuracy, which is the ability to obtain a desired beam shape and the ability to suppress sidelobes (gain responses in unwanted directions), is highly dependent on the precision and accuracy of the electronically controlled time delay or phase shift device in each element. The gain response of every antenna element must be precisely and accurately controllable and in wideband applications the frequency response characteristics must also be matched to achieve optimum results. As the operating frequency is increased the mechanical and electrical tolerances become critical, such that it has become impractical to fabricate antennas with the degree of process control required to achieve acceptable performance. This is particularly true for antennas operating at microwave frequencies.
One of the methods used in the manufacture of phased arrays is to place a small transmitting or receiving element in the near field of each element and to use it to measure the gain and phase or time delay characteristics of each individual element. A compensation table can then be generated and used to adjust the magnitude of the control voltages to correct for the gain and phase errors of individual elements. This process is time consuming and is typically only valid at the frequency and the ambient temperature at which the calibration measurements were made.
Other prior art examples use even more complex designs which provide an improved application-specific integrated circuit (ASIC) or a monolithic microwave integrated circuit with coupling. Further developments have used a combination of these prior art designs. See, for example, U.S. Pat. No. 6,163,296; U.S. Pat. No. 6,208,287; U.S. Pat. No. 6,252,542; and U.S. Pat. No. 6,480,153.